Enucleation or evisceration of the eye is performed because of disease or trauma that makes the removal of the eye, or the intraocular contents of the eye, necessary. Following either such procedure, the patient normally desires use of an artificial eye to restore a more normal appearance. The artificial eye is generally a large contact lens shaped object, which contains components that create the impression of the external aspects of an eye, i.e., an iris, pupil and sclera. To satisfactorily fit an artificial eye into the orbital socket, an orbital implant must be placed within the orbit to replace the volume that was lost when the eye or its contents was removed.
The use of an orbital implant and the subsequent fitting of the artificial eye confer more than a cosmetic benefit. They help maintain the normal structure of the eyelids and eyebrows; they aid in normal tear drainage; and, when used in children, they help stimulate normal growth of the orbital bones.
Even though an artificial eye can be made which has a very realistic appearance, prior to the present invention such artificial eyes have failed to track in conjunction with the normal eye because there was no coupling between the artificial eye and the orbital implant. The artificial eye drifted within the socket and did not track with the normal eye. This lack of tracking was quite apparent and disconcerting to even a casual observer, creating a sense of self-consciousness on the part of the patient.
Consequent to the shortcomings of traditional implants, efforts have been made to attach the eye muscles to the implant, and then to attach the artificial eye to the implant. This provided adequate tracking of the artificial eye. The success was short-lived because, in a brief period of time, the implant was extruded from the orbit. Implant extrusion occurred because the fixing of the artificial eye to the implant material exposed the implant to the outside environment. This permitted infectious agents such as bacteria to enter, and the implant became chronically infected. Exposure of these implants to the external environment was necessary to produce an attachment between the implant and the artificial eye.
A wide variety of materials have been used for orbital implants, such as ivory spheres, gold globes, silk, catgut, acrylic plastics or silicones, human bone (G. C. Sood al International Surgery, (1970) Vol. 54, No. 1, p. 1); and antigen-free cancellous calf bone, so called "Kiel Bone," (A. C. B. Molteno, et al., Brit. J. Ophthal., (1973) Vol. 57, p. 615 and A. C. B. Molteno, Trans: of the Ophthal. Soc. New Zealand (1980) Vol. 32, p. 36. These materials did not provide for significant integration of tissue and vascularization of the implant. As described in U.S. Pat. No. 4,976,731, these materials were disadvantageous in that the patient risked chronic infection as a result of subsequent procedures necessary to connect the implant to the artificial eye so that the artificial eye would track with the patient's contralateral eye. Also, the weight of the artificial eye was not supported by the implant. This lack of support puts pressure on the lower lid causing lower lid sagging.
A porous orbital implant overcomes these problems. One type of porous orbital implant is described in U.S. Pat. No. 4,976,731. In the 4,976,731 patent, the use and preparation of a porous orbital implant comprising hydroxyapatite is described. The use of porous implants allowed integration of the implant with fibrovascular tissue. Integrated implants provided advantages over other implant materials particularly because integration of the patient's own tissue allowed coupling of the implant to the artificial eye, and increased the long-term stability of both the artificial eye and the implant.
As disclosed in copending applications, e.g., Ser. Nos. 08/241,960 filed May 12, 1994; and, 08/660,095 filed Jun. 6, 1996, in order to couple an artificial eye to an integrated implant, the implant must generally be drilled (i.e., "tapped") so that a peg capable of connecting the implant to the artificial eye can be placed. It was necessary to pre-tap the integrated implants because the material used for the pegs lacked the strength to be inserted into the implant without pre-tapping. Pre-tapping, however, can sometimes be disadvantageous in that the surgeon must acquire additional equipment, e.g., a motorized drill. Furthermore, the use of a drill on delicate orbital tissue can lead to tissue trauma.
The need for self-tapping integrated orbital implant pegs is also related to the need to find more reliable implant materials to replace broken or deteriorating parts of the human body. Implant materials are needed in modern surgery and dentistry, such as metals and alloys, which are extremely chemically inert and which have adequate mechanical strength.
The first metal alloy developed specifically for human implant use was "vanadium steel", which was used to manufacture bone fracture plates (Sherman plates) and screws. Most metals such as iron (Fe), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), tantalum (Ta), molybdenum (Mo), and tungsten (W) used to make alloys for use in implants can be tolerated by the body in minute amounts. Sometimes those metallic elements, in naturally occurring forms, are essential in cell functions (Fe) or synthesis of vitamin B.sub.12 (Co) but cannot be tolerated in large amounts in the body. The biocompatibility of a metallic implant is of considerable concern because these implants can corrode in an in vivo environment. The consequence of corrosion are the disintegration of the implant material per se, which will weaken the implant, and the potentially harmful effect of corrosion products which escape into the surrounding tissue.
Metals and alloys in common use include stainless steels, Co--Ni--Cr alloy, cast and wrought Co--Cr--Mo alloy, commercially pure titanium, Ti-6A1-4V alloy and other titanium alloys. Biomaterial development is proceeding in the area of polymers, ceramics, combination materials such as zirconia-hydroxyapatite, and with aluminum oxide (alumina) which can be made with various degrees of porosity and strength. Key objectives with the development of these materials, as with metals and metal alloys, is biocompatability and strength.
Presently, commercially pure titanium is a material of choice for many implants, because of its biocompatibility resulting in no allergic reaction with the surrounding tissue and also no thrombotic reaction with the blood of the human body.
Stainless Steels
The first stainless steel utilized for implant fabrication was 18-8 (type 302 in modern classification), which is stronger and more resistant to corrosion than the vanadium steel. Vanadium steel is no longer used in implants, since its corrosion resistance is inadequate in vivo. Later 18-8sMo stainless steel, which contains a small percentage of molybdenum to improve the corrosion resistance in salt water, was introduced. This alloy became known as type 316 stainless steel. In the 1950s, the carbon content of 316 stainless steel was reduced from 0.08% (all are weight percentages unless specified) to 0.03% maximum for better corrosion resistance to chloride solution and became known as type 316L stainless steel. The minimum effective concentration of chromium is 11% to impart corrosion resistance in stainless steels. Chromium is a reactive element, but it and its alloys can be passivated to give excellent corrosion resistance.
CoCr Alloys
There are basically two types of cobalt-chromium alloys; one is the CoCrMo alloy which is usually used to cast a product and the other is CoNiCrMo alloy, which is usually wrought by (hot) forging. The two basic elements of the CoCr alloys form a solid solution of up to 65% Co. The molybdenum is added to produce finer grains which results in higher strengths after casting or forging.
The castable CoCrMo alloy has been used for many decades in dentistry and, recently, in making artificial joints. The wrought CoNiCrMo alloy is a relative newcomer now used for making the stems of prostheses of heavily loaded joints such as the knee and hip.
The CoNiCrMo alloy, originally called MP35N (Standard Pressed Steel Co.), contains approximately 35% Co and Ni each. The alloy is highly corrosion resistant to seawater (containing chloride ions) under stress. Cold working can increase the strength of the alloy considerably. However, there is considerable difficulty in cold working this alloy, especially when making large devices such as hip joint stems. Only hot-forging can be used to fabricate a large implant with the alloy.
The mechanical properties required for CoCr alloys are, as with other alloys, that increased strength is accompanied by decreased ductility. Both the cast and wrought CoCr alloys have excellent corrosion resistance.
Ti and Its Alloys
Titanium was discovered in 1794 and is the ninth most common element in the earth's crust, occurring as rutile, TiO2. Extraction of titanium in amounts that were large enough for commercialization came about with the developments of the Kroll process in 1936. Titanium has a high strength-to-weight ratio that makes it attractive for many applications. Attempts to use titanium for implant fabrication date from the late 1930s. It was found that titanium was tolerated in cat femurs, as was stainless steel and Vitallium (CoCrMo alloy). Commercially pure titanium and the common titanium alloy, Ti-6A1-4V, have been in use as implant materials for a shorter time compared with stainless steel and cast or wrought cobalt based alloys.
Titanium's lightness (4.5 g/cm.sup.3 compared to 7.9 g/cm.sup.3 for 316 stainless steel, 8.3 g/cm.sup.3 for cast CoCrMo, and 9.2 g/cm.sup.3 for wrought CoNiCrMo alloys) and good mechanochemical properties are salient features for implant application. Titanium alloys are prominent as dental and orthopedic implant materials because of their high strength-to-weight ratio, lower elastic modulus, excellent corrosion resistance and apparent biocompatibility.
Titanium and its alloys are used in orthopedic surgery as implants in the shape of wires, nails, plates and screws for the fixation and stabilization of fractures or in the form of artificial joints for the replacement of joints of the human body. Some implants are used for short time durations in the human body, whereas others remain in place for decades. To avoid a reoperation caused by the implant material, the material must meet certain chemical and mechanical requirements. Chemical requirements include high biocompatibility without altering the environment of the surrounding tissue even under deformation and sterilization. Mechanical property requirements relate to specific strength, modulus, fatigue, creep and fracture toughness which, in turn relate to microstructures.
To attain higher strength than commercially pure titanium, alloying elements are added. Alloy design criteria are not based only on alloying elements contribution to strength, but on the biocompatibility of the resulting alloy. Alloying additions and thermomechanical processing dictate the microstructure of the implant material, and control of microstructure is a means to attain desirable properties.
As the impurity content of commercially pure titanium becomes higher, there is increased strength and reduced ductility. The strength of the material varies from a value much lower than that of 316 stainless steel or the CoCr alloys, to a value about equal to that of annealed 316 stainless steel or the cast CoCrMo alloy. However, when compared by specific strength (strength per density), the titanium alloys excel relative to other implant materials. Titanium, nevertheless, has poor shear strength, making it less desirable for bone screws, plates, and similar applications. Titanium also tends to gall or seize when in sliding contact with itself or another metal.
Based on structures that can be produced by alloying, titanium alloys are grouped as alpha, alpha-beta and beta alloys. Alpha titanium and alpha-beta alloys have been used for dental and orthopedic purposes. Beta titanium alloys are being considered as candidate materials for implant applications because of their ease of formability, increased strength and lower elastic modulus, in spite of increased cost. Studies show the presence of the omega phase in the beta alloy, Ti-15Mo-2.8Nb, in the unaged condition. Comparison of corrosion behavior of this alloy with the alloy Ti-6A1-4V shows the two alloys have comparable corrosion resistance in simulated physiological solution.
Surface treatment variations such as porous coatings, ion implantation and oxidation are made to the titanium implant devices for various reasons; all directed to improving performance and biocompatibility. The use of "new" alloys and associated heat treatments and surface variations may result in changes in the mechanical and chemical behavior that ultimately affect the strength, durability and biocompatibility of the implant.
The physical properties of titanium alloys are affected by several parameters. Alloying elements and thermomechanical processing including shaping and sizing of the implants, affect the various aspects of mechanical properties in different ways. In general, increasing the strength by alloying or by thermomechanical processing decreases the fracture toughness of the material. Increasing the grain size is detrimental to fatigue behavior; however, creep resistance is increased. Cold working and hot working at relatively low temperatures develop a texture that, in turn, makes the mechanical behavior of the metal non-isotropic. Non-isotropic mechanical behavior may be useful in applications where directional properties are needed for improving reliability of the implant material.
As set forth above, integrated orbital implants allow vascularization of the implant itself. As a result of the shortcomings of prior implant pegs, there exists a need for means for attaching an artificial eye to an implant without the need for pre-drilling the implant.